Organic electroluminescence (EL) devices are much expected to be useful as inexpensive, large-sized full color display devices of solid state emission type and many developments have been made thereon. An organic EL device is generally constructed from a light emitting layer and a pair of opposite electrodes sandwiching the light emitting layer. When an electric field is applied between the electrodes, electrons are injected from a cathode and holes are injected from an anode into the light emitting layer. The injected electrons recombine with the injected holes in the light emitting layer to form excited states. When the excited states return to the ground state, the energy is released as light.
A phosphorescent organic EL device wherein a phosphorescent organic material is used in the light emitting layer has been proposed. Utilizing the singlet excited state and the triplet excited state of the phosphorescent organic material, a high emission efficiency can be obtained by the phosphorescent organic EL device. When electrons and holes are recombined in an organic EL device, singlet excitons and triplet excitons may generate in a ratio of 1:3 in accordance with their difference in the spin multiplicity. Therefore, an organic EL device employing the phosphorescent emitting material would achieve an emission efficiency three to four times higher than that of an organic EL device employing only the fluorescent emitting material.
The early organic EL device requires a high driving voltage and is insufficient in the emission efficiency and durability. To eliminate these problems, various technical improvements have been made.
The improved emission efficiency and the prolonged lifetime are very important for reducing the power consumption of displays and improving the durability. Therefore, further improvements have been still required. In addition, many studies have been made in order to improve the emission efficiency and the device lifetime of organic EL devices employing a phosphorescent emitting material.
A carbazole derivative has been used particularly as a phosphorescent host material because it has a high triplet energy. It has been also studied to use the carbazole derivative as a hole transporting material in the vicinity of a phosphorescent host, because the carbazole derivative makes the ionization potential (Ip) shallow to increase the hole transporting ability.
The carbazole derivative has been modified in its molecular structure. Patent Document 1 discloses a derivative having an indenoindole skeleton, Patent Document 2 discloses a derivative having an indenocarbazole skeleton, and Patent Document 3 discloses a derivative having an indolocarbazole skeleton.